The present invention relates to a method for sterilizing products to inactivate biological contaminants such as viruses, bacteria, yeasts, molds, mycoplasmas and parasites.
Several products that are prepared for human, veterinary or experimental use may contain unwanted and potentially dangerous contaminants such as viruses, bacteria, yeasts, molds, mycoplasmas and parasites. Consequently, it is of utmost importance that any biologically active contaminant in the product be inactivated before the product is used. This is especially critical when the product is to be administered directly to a patient, for example in blood transfusions, organ transplants and other forms of human therapies. This is also critical for various biotechnology products which are grown in media which contain various types of plasma and which may be subject to mycoplasma or other viral contaminants.
Previously, most procedures have involved methods that screen or test products for a particular contaminant rather than removal or inactivation of the contaminant from the product. Products that test positive for a contaminant are merely not used. Examples of screening procedures include the testing for a particular virus in human blood from blood donors. However, such procedures are not always reliable and are not able to detect the presence of viruses in very low numbers. This reduces the value or certainty of the test in view of the consequences associated with a false negative result. False negative results can be life threatening in certain cases, for example in the case of Acquired Immune Deficiency Syndrome (AIDS). Furthermore, in some instances it can take weeks, if not months, to determine whether or not the product is contaminated.
More recent efforts have focused on methods to remove or inactivate contaminants in the products. Such methods include heat treating, filtration and the addition of chemical inactivants or sensitizers to the product. Heat treatment requires that the product be heated to approximately 60xc2x0 C. for about 70 hours which can be damaging to sensitive products. Heat inactivation can destroy up to 50% of the biological activity of the product. Filtration involves filtering the product in order to physically remove contaminants. Unfortunately this method may also remove products that have a high molecular weight. Further, in certain cases small viruses may not be removed by the filter because of the larger molecular structure of the product. The procedure of chemical sensitization involves the addition of noxious agents which bind to the DNA/RNA of the virus and which are activated either by UV or ionizing radiation to produce free radicals which break the chemical bonds in the backbone of the DNA/RNA of the virus or complex it in such a way that the virus can no longer replicate. This procedure requires that unbound sensitizer is washed from cellular products since the sensitizers are toxic, if not mutagenic or carcinogenic, and can not be administered to a patient.
Irradiating a product with gamma irradiation is another method of sterilizing a product. Gamma irradiation is effective in destroying viruses and bacteria when given in high total doses. (Keathly, J. D. Et al.; Is There Life after Irradiation? Part 2; BioPharm July-August, 1993, and Leitman, Susan F.; Use of Blood Cell Irradiation in the Prevention of Post Transfusion Graft-vs-Host Disease; Transfusion Science 10:219-239, 1989). However, the published literature in this area teaches that gamma irradiation can be damaging to radiation sensitive products such as blood. In particular, it has been shown that high radiation doses are injurious to red cells, platelets and granulocytes (Leitman, ibid). Van Duzer, in U.S. Pat. No. 4,620,908 discloses that the product must be frozen prior to irradiation in order to maintain the viability of a protein product. Van Duzer concludes that:
xe2x80x9cIf the gamma irradiation were applied while the protein material was at, for example, ambient temperature, the material would be also completely destroyed, that is the activity of the material would be rendered so low as to be virtually ineffective.xe2x80x9d
Unfortunately, many sensitive biologicals, such as blood, would lose viability and activity if subjected to freezing for irradiation purposes and then thawing prior to administration to a patient.
In view of the above, there is a need to provide a method of sterilizing products that is effective in removing biological contaminants while at the same time having no adverse effect on the product. The present invention has shown that if the irradiation is delivered at a low dose rate, then sterilization can be achieved without harming the product. No prior references have taught or suggested that applying gamma irradiation at a low dose rate can overcome the problems admitted in the prior references.
Accordingly, the present invention provides a method for sterilizing a product comprising irradiating the product with gamma irradiation at a rate from about 0.1 kGy/hr. to about 3.0 kGy/hr. for a period of time sufficient to sterilize the product.
The rate of irradiation can be specifically from about 0.25 kGy/hr. to about 2.0 kGy/hr., more specifically from about 0.5 kGy/hr. to about 1.5 kGy/hr. and even more specifically from about 0.5 kGy/hr. to about 1.0 kGy/hr.
The term xe2x80x9csterilizexe2x80x9d as used in the present application generally means to inactivate any biological contaminant present in the product.
The length of time of irradiation or the total dose of irradiation delivered will depend on the bioburden of the product, the nature of the contaminant and the nature of the product.
Higher doses of irradiation are required to inactivate viruses as compared to bacteria. For example, using the dose rates of the present invention, one may use an irradiation time of greater than 10 hours to eliminate viral contamination in contrast to an irradiation time of only 45 minutes to remove bacterial contamination.
The process according to the present invention can be carried out at ambient temperature and does not require the heating, freezing, filtration or chemical treatment of the product before the process is carried out. This offers another significant advantage of the present process as it avoids some of the extra treatment steps of the prior art processes.
Certain products, such as blood, may be diluted prior to irradiation. Diluting the product may serve to reduce degradation of the product during irradiation. The choice of diluent depends on the nature of the product to be irradiated. For example, when irradiating blood cells one would choose a physiologically acceptable diluent such as citrate phosphate dextrose.
In cases where living cells (such as blood cells) are to be irradiated, a scavenger may be added to bind free radicals and other materials that are toxic to cells. A suitable scavenger is ethanol.
The efficacy of the method of the present invention is contrary to what others skilled in this area have observed or predicted. (U.S. Pat. No. 4,620,908 and Susan Leitman, ibid). The method provides a method of irradiating products that is not harmful to the product itself. In particular, the method of the present invention can effectively sterilize a product as fragile as blood without destroying the viability of the cells contained therein. Consequently the method of the present invention offers a significant technical and scientific advancement to the sterilization field. The method also provides an invaluable service to health care and the general public by providing a method to produce safe and sterile products for human use. This is especially critical in light of the spread of viral diseases such as AIDS and hepatitis through the transfusion of contaminated blood and blood products.